The present invention relates specifically to metal plastic laminates having high bond strength and improved resistance to acid undercutting during manufacture of laminates into flexible printed circuits.
The present invention further broadly relates to treating copper and copper alloys to form a tarnish and oxidation resistant film thereon.
In the manufacture of flexible printed circuits, copper foil or sheet is employed which normally has applied to its surface a film of an organic inhibitor, such as benzotriazole, in order to prolong the shelf life before manufacture of the circuit.
Flexible printed circuits comprise copper sheets or copper foil bonded to the surface of a plastic sheet, such as a polyester or polyimide. Normally two types of copper foil, either wrought or electro deposited, are employed in the manufacture of flexible printed circuits. Further it is advantageous to utilized wrought and annealed copper foil.
Printed circuits find wide use in the electrical and electronic fields since they are advantageous in the elimination of individual lead wires which require a separate soldering or other joining operation to the various components of any particular circuit. The configuration of such a circuit facilitates the positioning of conventional circuit components such as capacitors, etc., and the soldering of these components to the wiring by a dipping operation.
The manufacture of flexible printed circuits comprises adhesively bonding or laminating copper sheet or foil to a plastic film, such as a polyester or polyimide, and generally employing a suitable glue. As one preferred way the copper side of the resultant laminate is then sprayed with a photoresist and the required circuit is projected onto the resist-coated side of the copper component which transforms the photoresist into an acid insoluble compound in a figure and likeness of the circuit. The laminate is then immersed or sprayed with an acid etchant, such as a ferric chloride solution, to dissolve away the unwanted portion of the copper, i.e., that portion of the copper component of the laminate which is not part of the required circuitry.
Various problems arise however in the present manufacture of flexible printed circuits to which the present invention is directed.
For example, in order to provide tarnish resistance of rolled copper and an acceptable laminated product, before laminating a film of an organic inhibitor is normally applied to the surface of the copper.
The organic inhibitor, e.g., benzotriazole, provides for long shelf life or stability during storage.
Before laminating of a wrought hard copper to the plastic film it is advantageous to anneal the copper in order to provide increased ductility which is highly desirable in flexible printed circuits. It has been found that the organic inhibitor upon the copper surface decomposes during the annealing. Due to this decomposition problems arise such as the effect of the benzotriazole is no longer apparent and therefore the product no longer has good shelf life and tarnishing occurs. The tarnishing causes both poor laminate bond strength, uneven acid etching, and rapid acid undercutting along the bonded interface during etching away of the unwanted copper portion of the laminate. The acid undercutting generally occurs at a rate equivalent to at least thirty mils per hour from each side of the copper circuitry, at the aforementioned interface, and materially degrades the quality of the printed circuit.
Therefore, unless treated the copper foil-plastic laminate exhibits poor bond strength when room temperature oxidation or tarnishing occurs on the foil. Furthermore, the resistance to acid undercutting along the interface of the wrought annealed foil is poor as aforementioned. A further complication with wrought annealed and other foil arises with the use of organic inhibitors such as benzotriazole, since residual benzotriazole on the unbonded side of the foil results in uneven etching of the circuit because the benzotriazole provides some inhibition in the etching solution. A still further disadvantage with organic inhibitors occurs with certain plastic systems wherein high temperatures, i.e., above 240.degree.F, are employed for curing of the glue. These high temperatures cause the copper-organic inhibiting film to decompose with the formation of relatively large amounts of gases which causes blistering of the laminate and thereby producing an unacceptable product.
It is well known, as aforementioned, that copper and many of its alloys possess low resistance to tarnishing in many atmospheres and particularly atmospheres containing industrial wastes such as compounds of sulfur. It is therefore required, in order to provide a measure of tarnish resistance for a relatively prolonged period of time, that a film of an organic inhibitor, such as benzotriazole, be applied to the surface of the copper or copper alloy.
The application of such inhibitors provides for prolonging the aesthetic appeal of copper materials in finished form, such as lamp bases and other consumer goods for the home, and also provides for long shelf life before further manufacture of such materials into final consumer articles. This is particularly important since prolonged exposure of copper materials in an industrial environment naturally degrades the exposed surfaces resulting in build up of corrosion products, such as copper oxides and sulfides, which may necessitate a severe mechanical or chemical cleaning operation in order to restore the material surfaces to a condition compatible for normal further cleaning and manufacturing operations, e.g., a simple alkaline clean or degreasing cycle before additional mechanical working or soldering of the material.
It is therefore a principal object of the present invention to provide a method for producing increased tarnish resistance of copper and its alloys, and the article produced thereby. It is an additional object of the present invention to provide a method for producing a copper, or copper alloy sheet or foil adhesively laminated to a plastic film to form a laminate wherein the laminate is characterized by increased resistance to acid undercutting and uneven dissolution of the unwanted copper during manufacture of the laminate into a flexible circuit, and the article produced thereby.
It is a further object to provide a method for producing a flexible printed circuit which is characterized by no substantial undercutting of the circuitry and by high bond or peel strength and tarnish resistance without degradation of other properties so desirable in flexible printed circuitry, and the article produced thereby.
It is still a further object of the present invention to provide the objects as aforesaid conveniently, expeditiously and inexpensively.
Further objects and advantages of the present invention will become apparent hereinafter.